1. Field of the Invention
The present invention relates to a method for producing a grain-oriented electrical steel sheet. More particularly, the present invention relates to a method for producing a grain-oriented electrical steel sheet having a high magnetic flux density, by utilizing completely novel precipitates which are effective for generating the secondary recrystallization which is used as a fundamental metallurgical phenomenon for the grain-orientation. Such precipitates are referred to as the inhibitors.
2. Description of the Related Arts
Grain-oriented electrical steel sheet consists of crystal grains having the Goss orientation (expressed by the Miller index as a {110} &lt;001&gt; orientation), in which the {110} plane is parallel to the surface of a steel sheet and the &lt;100&gt; axis coincides the rolling direction. The grain-oriented electrical steel sheet is used as the core of a transformer and a generator, and must have good exciting characteristics and watt loss characteristics. The quality of the exciting characteristics is determined by the magnitude of a magnetic flux density induced in the core at a constant magnetizing force applied to the core. A high magnetic flux density is attained by aligning the orientation of crystal grains to {110} &lt;001&gt; at a high degree. The watt loss is a loss of power consumed as thermal energy when the core is energized by a predetermined alternating magnetic field. The quality of watt loss is influenced by magnetic flux density, sheet thickness, quantity of impurity, resistivity, grain size, and the like. Particularly, a grain-oriented electrical steel sheet having a high magnetic flux density is preferred, since the size of electrical appliances as well as the watt loss can be accordingly lessened.
Note, the grain-oriented electrical steel sheet is obtained by means of reducing the sheet thickness to a final thickness by an appropriate combination of hot-rolling, cold-rolling, and annealing, and by means of a subsequent, finishing high-temperature annealing, in which the primary recrystallized grains having {110} &lt;001&gt; orientation are caused to selectively grow, that is, a secondary recrystallization is effected. The secondary recrystallization is attained, when fine precipitates, such as MnS, AlN, MnSe, and the like, or an element present in the grain-boundary (hereinafter "grain-boundary element") such as Sn, S, P, and the like, are preliminarily present in the steel. As described by J.E. May and Turnbull in Trans. Met. Soc. AIME Vol. 212 (1958) pages 769/781, the precipitates and grain-boundary elements have functions, during the finishing high-temperature annealing, for suppressing a growth of primary recrystallized grains having orientations other than {110} &lt;001&gt; and causing a selective growth of those having {110} &lt;001&gt; orientation. The suppression of the crystal growth as described above is generally referred to as the inhibitor effect. Accordingly, researchers in the relevant technical field have stressed the study of the kind of precipitates or grain-boundary elements to be used to stabilize the secondary recrystallization and how to attain an appropriate existence state thereof for enhancing the proportion of accurate {110} &lt;001&gt; oriented grains.
With regard to the kinds of precipitates, the following disclosures have been published. M.F. Littmann in Japanese Examined Patent Publication No. 30-3651 and May and Turnbull in Transactions Metallurgical Society AIME 212 (1958) p 769/781, disclosed MnS; Taguchi and Sakakura disclosed AlN in Japanese Examined Patent Publication No. 33-4710; Fiedler disclosed VN in Transactions Metallurgical Society AIME 221 (1961) p 1201/1205; Imanaka disclosed MnSe in Japanese Examined Patent Publication No. 51-13469; and, Fast disclosed Si.sub.3 N.sub.4 in Philips Search Report (1956) 11, p 490. In addition, TiS, CrS, CrC, NbC, SiO.sub.2, and the like have been disclosed.
With regard to the grain boundary elements, As, Sn, Sb and the like are described in TRANSACTIONS of JAPAN INSTITUTE OF METALS 27 (1963) p 186 (Tatsuo Saito). In industrial production, the grain boundary elements are not used above but in the presence of precipitates, in an attempt to realize a supplement effect of the precipitates. For a stable industrial production of a grain-oriented electrical steel sheet and an alignment of {110} &lt;001&gt; orientation at a high degree, a solution is sought by determining which kinds of precipitates are to be utilized.
A criterion for selecting precipitates effective for the secondary recrystallization has not been satisfactorily elucidated. The opinion of Matsuoka described in Tetsu To Hagane 53 (1967) p 1007/1023 is representative of such criterion, and is summarized as follows.
(1) Size of approximately 0.1 .mu.m PA1 (2) Necessary volume of 0.1 vol% or more PA1 (3) Neither complete solution nor complete non-solution at a temperature range of secondary recrystallization are admitted. Precipitates need to solid dissolve at an appropriate degree. PA1 (1) Majority of constitution elements of the precipitates are Si, which is present in the steel in a large amount, as well as Al, which is added to the steel in a small amount. Therefore, it is not necessary to add expensive elements so as form the precipitates, and it is easy to attain by an inexpensive means the formation of precipitates in a large amount. PA1 (2) The solid-dissolving temperature of the precipitates is high. The precipitates, therefore, do not undergo a morphology change until the temperature is elevated to a considerable high level in the finishing high-temperature annealing. The precipitates can, therefore, contribute to the generation of a stable secondary recrystallization and to the growth of grains having an orientation close to the {110} &lt;001&gt; orientation. PA1 (3) The precipitates can be formed by a very simple method. That is, the steel sheet is nitrided from outside at an intermediate step of the production process, for treating the steel containing a minute amount of solute Al. The precipitation amount can be easily controlled since the nitrogen is given to steel from the exterior thereof. PA1 (A) B.sub.10 = 1.95 Tesla, W.sub.17/50 = 0.75 w/kg. PA1 (B) B.sub.10 = 1.87 Tesla, W.sub.17/50 = 1.12 w/kg.
The above various precipitates satisfy the above requirements. As is apparent from the above summary, a large amount of fine precipitates must be present uniformly in the steel sheet prior to the finishing high-temperature annealing, so as to obtain a high alignment degree of {110} &lt;001&gt; orientation, and hence a high magnetic flux density. A number of techniques, in which components of a starting material and the conditions for heat treatment are controlled have been developed for forming such precipitates. For obtaining materials having a high magnetic flux density, it is important to control the precipitates, and in addition, to control the properties of the primary recrystallized structure by means of an appropriate combination of heat treatment, in such a manner that the recrystallized structure is adapted to the precipitates.
The grain-oriented electrical steel sheets are produced industrially, at present, by the three representative methods, all of which involve significant problems.
The first method is the dual cold-rolling method using MnS, disclosed in Japanese Examined Patent Publication No. 30-3651 by M.F. Littmann. The second method is disclosed in Japanese Examined Patent Publication No. 40-15644 by Taguchi and Sakakura, and is characterized by a heavy cold-rolling of 80% or more at the final cold-rolling and by using AlN + MnS. The third method is disclosed in Japanese Examined Patent Publication No. 51-13469 and is characterized by a double cold-rolling process with the use of MnS and/or MnSe + Sb. In all of the above methods, the heating of a slab prior to hot-rolling is carried out at a high temperature, so as to control the precipitates to be fine and uniform, such that: the slab-heating temperature employed in the first method is 1,260.degree. C. or more; although dependent upon the Si content of the starting material, 1,350.degree. C. is employed in the second method as described in Japanese Unexamined Patent Publication No. 48-51852; and, in the third method, as is described in Japanese Unexamined Patent Publication No. 51-20716, 1,230.degree. C. or more is employed, and even 1,320.degree. C. is employed in an example in which the high magnetic flux density is attained by means of dissolving the precipitates, once formed coarsely at an extremely high temperature, such as 1,320.degree. C., into a solid solution of Si steel and then finely precipitating them during the hot-rolling or heat treatment. A high temperature heating for the slabs incurs the following problems: Energy used for heating the slabs is increased; Slags are formed, and have the yield is lessened and repairing expenses are increased. In addition, as disclosed in Japanese Examined Patent Publication No. 57-41526, a failure of the secondary recrystallization is generated when continuous cast slabs are used, that is, these slabs cannot be used for producing grain-oriented electrical steel sheets. Furthermore, as disclosed in Japanese Examined Patent Publication No. 59-7768, the failure of the secondary recrystallization mentioned above becomes more serious when the sheet thickness is further reduced.
The above methods involve further problems. In the first method, a high magnetic flux density is obtained with difficulty, and B.sub.10 only amounts to approximately 1.86 Tesla. In the second method, appropriate production-conditions are narrowly limited in implementing industrial production, and therefore, the second method fails to stably produce products having the highest magnetic properties. The production cost is high in the third method, because it uses a double cold-rolling method and uses harmful and expensive elements, such as Sb and Se. The above methods also involve more essential and important problems than those described above. That is, in these methods, the magnetic flux density is restricted by the greatest volume of precipitates, which can be uniformly formed by these methods. More specifically, the constituting elements of the precipitates can be contained only within the solid solubility, under which the constituting elements are caused to dissolve into the solid solution of silicon steel. A method for enhancing the magnetic flux density by increasing the quantity of precipitates can therefore be carried out as long as such quantity is kept under the solid-solubility limit at slab heating.